Signal detecting system



Nov. 25, 1941.

w. R. KOCH I 2, SIGNAL bETECTING SYSTEM I Filed Jan. 31, 1940 3 Sheets-Sheet l fl 16 14 Ill in ield 1i.

Nov. 25, 1941.

w. R. KOCH SIGNAL DETECTING SYSTEM Filed Jan. 51,

1940 3 Sheets-Sheet 2 H FREQUENCY ield R. KOGZL W G ttorneg Nov. 25, 1941. w. R. KOCH SIGNAL DETECTING SYSTEM 3 Sheets-Sheet 5 Filed Jan. 31, 1940 31mm or Winfield Howl (Ittomcu Patented Nov. 25, 1941 SI NAL DETECTING SYSTEM Winfield R. Koch, Haddonfield, N. J., assignor to Radio Corporation of America, a corporation of Delaware Application January 31, 1940, Serial No. 316,622

Claims.

This invention relates to systems for detecting signals of the frequency modulated type, and has for its principal object the provision of an improved system and method of operation whereby the receiver is silenced when it is mistuned or when no signal is received.

A further object of the invention is the provision of a frequency modulated signal detecting system which rejects signals of the amplitude modulated type and responds to frequency modulated signals only when tuned to the midfrequency of such signals.

When a frequency modulated signal receiver of the type, disclosed by U. S. Patent 2,121,103, for example, is tuned through a signal, audible output is heard at three points corresponding to the mid-frequency and to frequencies on the nonadjacent sides ofthe voltage frequency characteristics of the discriminator or detector diodes. At frequencies other than the mid-band frequency, amplitude modulated signals much distorted by the receiverlimiter are heard as undesired noise. When no signal is present, stray noise due to various causes is also'heard. V

In accordance with the present invention, noise due both to mistuning of the receiver during signal reception, and to other causes in the'absence of signal, is suppressed through automatic variation in the grid bias potential of the audio amplifier of the receiver. To this end, the discriminator output voltage is utilized to bias oiT the audio amplifier when the receiver is mistuned to a received signal, and the potential of the common lead of the discriminator diodes is similarly utilized when no signal is received.

The invention will be better understood from the following description considered in connection with the accompanying drawings, and its scope is indicated by the appended claims.

Referring to the drawings,

Figure 1 is a wiring diagram of a frequency modulated detecting system provided with means for suppressing noise due to mistuning of the receiver,

Figure 2 is a wiring diagram of a detecting system including means for suppressing noise both in the presence and in the absence of a signal,

Figures 3 and 4 illustrate different modifications of the system of Fig. 2, and

Figure 5 is an explanatory diagram relating to the operation of the systems of the preceding figures.

The system of Fig. 1 includes an intermediate frequency amplifier l0, provided with a tuned input circuit l and with a tuned output circuit [2 which is coupled through a similarly tuned circuit l3 to the diode detectors 14-45 of the discriminating or detecting network by which the frequency modulation is converted to corresponding variations in voltage oramplitude.

The discriminating or detecting network is of a well known type and depends for its operation on the fact that the phase relation between the voltages of the tuned primary and secondary circuits of a coupling transformer changes with change in frequency. As pointed out in the aforesaid U.,S. Patent 2,121,103, the action depends upon the fact that a 90 phase difference exists between the primary and secondary potentials of a double tuned transformer when en ergy of the resonant frequency is applied, and this phase angle varies as the applied frequency varies. This type of network, therefore, afiords one satisfactory means of changing frequency modulated signals into corresponding amplitude modulated signals. Other means for producing the result are well known and obviously may be substituted for the particular means illustrated.

The illustrated detector network includes the tuned primary and secondary circuits l2-l3, the diodes I4l5, a capacitor l6 for coupling the high voltage side of the primary circuit to the mid-point of the secondary circuit, and resistors l1 and 18 which are series connected between the cathodes of the diodes I l-l5, are provided with a common terminal connected through a choke coil IE to the mid-tap of the secondary circuit 13, and are respectively bypassed by the capacitors 20-2l.

The detected modulation voltage is applied through a coupling capacitor 22,to the control electrode 6 of an audio frequency amplifier 23 provided with an input circuit which includes resistors 24 and 25 and a self-bias resistor 26 shunted by a by-pass capacitor 21, and with an output circuit which includes a resistor 28.

For applying to the audio frequency input circuit resistor25 a potential which is responsive to mistuning of the receiver, there are provided a suppressor device 29 and a double diode device 30. The device 29 is provided with a screen grid 1 and two control grids 8 and 9. The input circuit of the device 29 connected with the grid 9 mayinclude a resistor 3|, the resistors I1 8, the lower section of a bleeder resistor 32 and a selfbias or cathode lead resistor 33 which is shunted by a capacitor 34. Resistor 3| offers a high impedance to audio frequency currents, to permit only the D. C. variation to operate the suppressor device 29. The anode circuit of the device 29 includes a resistor 35, an intermediate section of the bleeder resistor 32, and the cathode lead resistor 33. The screen grid circuit of the device 29 includes a resistor 36, the intermediate section of the blecder resistor 32, and the cathode lead resistor 33.

The audio amplifier grid bias resistor 25 is connected at one end to a high voltage terminal of the bleeder resistor 32, and at the other end to a low voltage terminal of this resistor through either (1) the upper diode 3'! and the anodecathode circuit of the device 29, or (2) the lower diode 38 and the screen-grid-cathode circuit of the device 29.

How the noise usually incident to mistuning of the receiver is automatically suppressed will be readily understood. When the receiver is tuned to the mid-frequency F (Fig. the voltages of the diodes I 4 and I5 are equal and opposite, no additional voltages are applied to the control grid 9 of the device 29, the bias potential of the resistor is a minimum, and the detected modulation passes to the audio frequency amplifier 23. This follows from the fact that the potential drops of the resistors and 36 are made substantially equal and of such value that the diodes 31 and 38 are inoperative due to their cathodes being positive with respect to their anodes. Under these conditions, normal bias is applied to the control grid of the amplifier 23.

When the receiver is mistuned, however, the outputs of the diodes I4 and I5 are not equal and there is impressed on the control grid of the device 29 an additional potential which is either positive or negative, depending on which side of 5 the mid-frequency the receiver is mistuned.

If the additional voltage is of positive polarity, the potential drop of the resistor 35 increases, due to'the increased anode current of the device 23, the cathode of the diode 3'! becomes more negative, and current is drawn through the resistor 25, thus biasing the amplifier 23 to cut-off. Similarly, if the additional voltage applied to the control grid of the suppressor device 29 is of negative polarity, is increased, the cathode of the diode 38 becomes more negative, and current is drawn through the resistor 25, thereby biasing the amplifier 23 to cut off.

The system of Fig. 2 is similar to that of Fig. 1 in several respects, as indicated by the corresponding reference numerals, but differs therefrom in that (1) the connection between the primary and secondary circuits I 2 and I 3 is made through a capacity divider 3340 and (2) the common cathode-anode circuit of the detector diodes I4I-I5 is connected through the audiofrequency attenuating resistor 4| to the control grid 32 of a second suppressor device 42, which operates through a resistor 43, a diode i i, and the resistor 25 to bias off the amplifier 23 in the absence of a signal, to prevent passage or amplification of stray noise voltages, and (3) the control of amplifier 23 by applying the biasingoif voltage to a different grid 31 from that to which the audio signal is applied. Radio frequency currents are, of course, by-passed through the capacitors.

The operation of the system of Fig. 2.to sup press noise in the absence of signal is dependent on the potential applied through the resistor 4! to the control grid 32 of the device 42. Thus, lack of potential drop in resistor I8, when no appreciable signal is present, drop'in the resistor 43,

produces a large potential makes the cathode of the the potential drop of the resistor 36 ode. The space current through this tube will be large, making the voltage drop in the resistors in both the screen grid and plate circuits large enough to cause the diode 30 to become operative and bias of? the amplifier 23. When a strong signal is tuned in, the drop in resistor I8 becomes large, tending to make the first grid 8 of the suppressor device 23 negative, thus reducing the space current, and rendering the amplifier 23 operative. The diode 41 keeps the grid 8 from becoming more negative than the normal operating value, thus permitting the tube to be controlled by the third grid 9 when signals of the desired strength are being received.

The operation of this modification is similar to those of Figs. 1 and 2, and Will be readily understood Without detailed explanation.

The system of Fig. 4 differs from those of the preceding figures in that the suppressor device and its associated diodes are replaced by a fiipfiop circuit which is of the multivibrator type disclosed in U. S. Patent 2,050,059. This circuit functions through the resistor 25 to bias off the amplifier 23 in response to potentials resulting from mistuning and stray currents. It includes the resistor 3| through which potential is applied to the first grid 56 of a pentode 43, a resistor 49 through which the output of the device 48 is coupled to the second grid 58 of a pentode 50, and the resistor 25 through which the output of the device is coupled (1) to the audio frequency amplifier grid circuit for biasing off the amplifier 23 in response to mistuning potential and (2) to the second grid 59 of the device 48 for transferring current to this device in the absence of potential resulting from mistuning, and for securing the sudden and complete operation of the suppressing action, without any partially biased off condition of amplifier 23.

More specifically stated, the operation of the circuit in response to mistuning is as follows: When a signal is tuned in, correctly or incorrectly, the drop in potential across resistor I8 will bias off tube 5I, resulting in absence of any voltage drop in resistor 54. When a signal is tuned in correctly, the voltage drop in resistor II will be equal and opposite to that in resistor I8, and the control grids of both pentodes es and 55 will be at ground or chassis potential. Because of the drop across resistor 55 in the cathode circuit of tube 50, the effective bias of the control grid 60 of this tube will be larger than that of the control grid 56 of tube 48. As shown in the patent referred to, this results in a large current through tube 48, while that of tube 55 will be cut off. The drop through resistor 25 will be small, and the amplifier 23 will be operative. When a signal is mistuned somewhat, the control grids of tubes 13 and 55 will tend to become either more positive or more negative. The diode 52 prevents the control grid of tube 28 from becoming more positive, while the diode 53 prevents the control grid of tube 50 from becoming more negative, by becoming conductive under these conditions. It

remains, therefore, that the control grid of tube 63 may become more negative, while that of tube 55 may become more positive. When mistuning causes either of these two actions to occur, the tube 56 will tend to start passing current, and tube it! will begin passing less current, the action will be accumulative and tube 50 will conduct a large current, while tube 48 will be cut ofi. The large drop through resistor 25 will bias off the amplifier 23, rendering the receiver silent.

For biasing off the amplifier 23, to render the receiver silent to stray currents produced when no signal is present, there are provided a coupling tube 5|, the diode 53, and a resistor 54.

This function is performed as follows: Absence of an appreciable signal will result in no voltage drop across resistor l8, thereby removing the bias from tube 5|, which will then pass some current, resulting in a voltage drop across resistor 54. The diode 53 will become active, making the control grid 60 of tube 50 more positive. This initiates the reversing action of tubes 48 and '50, just as when the signal was mistuned so as to make this grid more positive, and the amplifier 23 rendered inactive.

Fig. 5 shows the drop in the diode resistors Ill and It as the frequency of the received signal is varied. Frequency modulated signals can be heard at three places, designated F, F1 and F2.

Positions F1 and F2 give a poor signal to noise ratio and result in considerable distortion. The invention prevents accidental tuning to either of these two positions by rendering the receiver silent under all conditions except when the carrier is tuned in the region near F.

I claim as my invention:

1. In a system for detecting signals of the frequency modulated type, the combination of a tunable circuit, an amplifier, an electron discharge device provided with a control electrode, a frequency discriminator network and rectifier means connected with said electrode for applying thereto potential resulting from mistuning of said circuit, anode and screen grid circuits for said device, and means connected in said i anode and screen grid circuits and with said amplifier for interrupting the operation of said amplifier.

2. In a system for detecting signals of the frequency modulated type, the combination of a tunable circuit, an amplifier, an electron discharge device provided with a control electrode, a frequency discriminator network and rectifier means connected with said electrode for applying thereto potential resulting from mistuning of said circuit, anode and screen grid circuits, and means including a pair of rectifiers having an output circuit connected with said amplifier and biasing resistors for said rectifiers connected in said anode and screen grid circuits for interrupting the operation of said amplifier.

frequency-modulated type, the combination of a tunable signal circuit, a signal amplifier responsive to a predetermined biasing potential for interrupting signals through said system, an electron discharge device provided with control electrode, anode and screen grid circuits, means for deriving from said system and applying to said control electrode circuit a potential resulting from 'mistuning of said circuit which varies in magnitude and polarity with variations in frequency about resonance, separate resistors connected in said anode and screen grid circuits for developing potentials responsive to said variations in the last-named potential, a pair of rectifier elements connected with said control electrode circuit responsive to predetermined values of said potentials across said resistors for establishing the first-named biasing potential in said circuit, and means connected between said first-named potential-deriving means and said control electrode circuit for controlling the operation of said amplifier in the absence of signals.

5. In a system for detecting signals of the frequency-modulated type, the combination of a tunable frequency-responsive discriminator network having a balanced output circuit providing controlling potentials which vary in magnitude and polarity with frequency variation of a received signal from resonance, a control amplifier connected with said output circuit for receiving a controlling potential therefrom, said amplifier having a pair of output circuits includ ing impedance elements providing potentials across said elements which vary with variations in controlling potential from said discriminator network, an audio frequency amplifier connected to receive signals from said discriminator network, said amplifier having a control grid circuit including a resistor element therein, a pair of rectifiers connected in circuit with said resistor element and connected with said impedance elements to receive biasing potentials therefrom whereby the operation of said rectifiers is prevented in the presence of an applied signal at the resonant frequency of said network.

WINFIELD R. KOCH. 

